Call redirection, which includes call forwarding and call coverage from one endpoint to another, is a common feature of many telecommunications switches, particularly private branch exchanges (PBXs). An illustrative implementation of a call coverage feature is described in U.S. Pat. No. 4,436,962. Over time, efforts have been made to extend features across entire networks of routing nodes (e.g., switches) to make features that work with intra-node calls also work with inter-node calls. This is illustrated, for example, in U.S. Pat. No. 4,488,004. The call coverage feature has been a specific focus of some of these efforts, as illustrated by U.S. Pat. Nos. 5,313,459 and 5,369,695, for example.
Call redirection typically provides a covered end-point with the option of specifying a sequence of alternative end-points (also called covering end-points) to which the call may be directed if it cannot successfully be completed to the first or any subsequent covering end-point in the list of covering end-points stored in memory, i.e., the coverage path. However, when the coverage path specifies a covering end-point that is served by another switch, such as an end-point reached by using the public switched network, the switch to which the call was initially directed (hereinafter, the principal's switch), normally relinquishes control of the call to the other switch. Therefore, if the principal's switch redirects a call for coverage to a covering remote end-point but the end-point is unable to accept the call, the principal's switch is not able to regain control of the call and redirect it to the next covering end-point specified in the coverage path. This is especially true when the covering switch is in the public telecommunications network or when the switches do not have a cooperating protocol in place.
Heretofore it has been appreciated that if there is a common channel signaling network linking the switches it may be employed to allow the subsequent covering switch to inform the previous (e.g., covered) switch whether or not the call can be completed and, if not, the first switch may resume call coverage.
Another approach is disclosed in the copending application of D. W. Herrick et al, U.S. Pat. No. 5,521,970 in which the switch originally handling the call determines whether the covering switch to which the call would be redirected is a "cooperating switch", i.e., one which has been configured to effect network call coverage like the original switch and with which it can exchange signaling control information, sometimes hereinafter referred to as "DCS". If the covering switch is not a "cooperating" switch, the first switch relinquishes control to the covering switch. If the covering switch is a "cooperating" switch, the first switch sends a data message to the covering switch identifying the call as a network coverage call, the second switch determines whether it has a call-covering end-point available to receive the call, signals that fact to the first switch and simultaneously rings the call-covering end-point. If the call covering end-point was not available at the end-point, the first switch resumes control of the call.
While the ability to immediately complete the call to the covering end-point in the above-identified Herrick et al patent avoided the problem of the covering end-point becoming unavailable, it would be extremely advantageous if call coverage could be performed "off-net", i.e., when the call covering end-point is located beyond the direct reach of the first switch to which the call was initially directed, without requiring a signaling channel between the two switches handling the call. In addition, it would be helpful if a call could be extended using any type of available voice-frequency or ISDN type trunk path linking the switches.